The present invention relates to temperature measurement, and in particular, to calibration of temperature measurement points.
Temperature measurement points are used in the process control industry to sense the temperature of a process fluid, for example. A temperature measurement point typically includes a temperature sensor connected to a transmitter. The transmitter is often located in a remote location and may be coupled to a control room over a 4-20 mA current loop, a digital communication bus, or a wireless network. The temperature sensor is placed in thermal contact with the process fluid and provides an output related to temperature of the process fluid. The temperature sensor, for example, may be a resistance temperature detector (RTD) which is a device having a temperature dependent resistance or a thermocouple which is a device producing a temperature dependent voltage. For a typical RTD, the transmitter injects a current into the RTD, and the resultant voltage across the RTD is used to measure resistance. The voltage is converted into a digital format using an analog-to-digital converter and provided to measurement circuitry in the transmitter. The measurement circuitry converts the measured voltage into a digital value representative of temperature.
In some cases, the process fluid may be an environment that is hazardous to the temperature sensor. In such situations, the temperature sensor is inserted into a thermowell, which is inserted into the process fluid. Thermowells are typically long, slender tubes or wells, open at one end to allow insertion of the temperature sensor and closed at the tip end to protect the temperature sensor from having direct contact with the process fluid.
Even when a thermowell is used, the relationship between resistance of an RTD and temperature tends to change over time. These changes can be periodically calibrated out of the system to maintain accuracy of the temperature measurement. For example, periodically an operator (or user) may be required to journey into the field to calibrate the temperature sensor. The temperature sensor is calibrated by taking it to a laboratory, placing the RTD sensor in a bath of a known temperature, and monitoring the measured temperature output from a calibrated meter. The difference between the actual temperature of the temperature bath and the measured temperature output is used as a calibration factor, entered into memory of the transmitter, and stored for subsequent use by the measurement circuitry. Similar calibration is performed for thermocouples and other temperature sensors.
Much like the temperature sensor, accuracy of the transmitter can also change over time. The transmitter is typically calibrated by disconnecting it from the temperature sensor and connecting it to a reference calibration tool. The reference calibration tool typically produces a known electrical parameter, such as resistance. The difference between the expected resistance and the measured resistance is used as a calibration factor and stored in memory for subsequent use by the measurement circuitry.
Unfortunately, these calibration techniques can be time consuming. Moreover, such calibration requires the transmitter to be offline for a period of time, not measuring the process fluid. In certain situations, the entire process is shut down until all temperature measurement points are back on line. Thus, calibration can be expensive, discouraging users from calibrating temperature measurement points as often as they should.